Precision Machining

The work included in this book focuses on precision machining and grinding processes, including milling, laser machining and polishing on various materials for high-end applications. These processes are in the forefront of contemporary technology, with significant industrial applications. Their importance is also made clear by the important works that are included in the research that is presented in the book. Some important aspects of these processes are investigated, and process parameters are optimized. This is performed in the presented works with significant experimental and modelling work, incorporating modern tools of analysis and measurements

Elements for the design of precision machine tools and their application to a prototype 450mm Si-wafer grinder

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 155-160).Next generation precision machines will require ever more rigid elements to achieve the required machining tolerances. The presented work focuses on the application of ultra stiff servo-controllable kinematic couplings and hydrostatic bearings to minimize the structural loop of multi-axis precision grinding machines while reducing complexity. The fundamental importance of these ultra stiff, adjustable machine elements is demonstrated in the design of a grinding machine for 450mm diameter silicon wafers. A new generation of silicon wafer grinding machines is needed to back-grind large (450mm diameter) wafers from the production thickness of up to 1 mm down to less than 50pm so as to reduce the cost of Si-wafer based components. The grinding process needs to be done in about 90 sec (fine-grinding, e.g. -200 micron) to 160 sec (coarse grinding, e.g. -600 micron). After completion of the fine grinding process the wafer must be flat to 0.1 pm/o45mm and parallel to 0.6pm/450mm diameter. The surface roughness must be less than Rymax 0.1 pm and Ra 0.01 pm. Even though the required machining forces are 1 N/nm is required, which is many times stiffer than a typical machine tool (0.1 to 0.3 N/nm). In cooperation with industry, this work had the aim of creating a new machine design philosophy, with an example application that focuses on nano-adjustable kinematic coupling and feedback controlled water hydrostatic bearing technology. This new design philosophy is needed to enable the design of a relatively small footprint, compact precision machines. In particular, a ball screw preloaded height adjustable kinematic coupling and a magnetically preloaded hydrostatic thrust bearing were designed and built. The adjustable kinematic coupling allows for up to 8mm of vertical height adjust and 7N/nm stiffness at 26 kN preload. By varying the preload on the coupling by +/- 10%, in-process nm to micron height and tilt adjustment at >95% of the nominal stiffness is possible. Under the assumption of a constant flow supply, the hydrostatic bearing achieves a theoretical stiffness of 1 N/ nm at a 20 micron bearing gap and 7000 N combined gravitational and magnetic preload. In practice, the stiffness is limited by the pressure flow characteristics of the supplying pumps. To increase the bearing stiffness to a required 4N/ nm, various control loops have been developed and tested.by Gerald Rothenhöfer.Ph.D

Development of the thermo-mechanical finite element model of the rotating units

Department of Mechanical EngineeringHigh-speed machining is a particularly important industrial process during which material is removed using a cutting tool with rotational and translational motion. The rotating unit is a major part of the machine tool, and is composed of a bearing, spindle, and associated surrounding components. During machining, the rotating unit is exposed to large mechanical loads, as well as significant heat generated due to friction in the bearings, which can lead to thermal deformation and failure of the spindle. To achieve reliability of the spindle system, an accurate description of the thermo-mechanical behavior of the system is useful to predict the thermal effects during the design phase. Existing methods to simulate the thermo-mechanical behavior of rotating parts require significant computational resources because of the geometric complexity of the models. In this dissertation we discuss a finite element method (FEM) approach for calculating the thermo-mechanical behavior of the rotating unit using the commercially available software package ANSYS. We modeled the heat generated in angular contact ball bearings, and applied the heat transfer of a rotating cylinder to calculate thermal load. We explored the characteristics of the spindle system, specifically angular contact ball bearings, which are commonly used in machine tools. We carried out a geometric simplification process for angular contact ball bearings out to overcome the divergence phenomenon in the FEM simulation, which may occur because of contact between rolling elements and bearing races. We used Matrix27 elements to simulate the stiffness/damping characteristics of ball bearings, and used the ???close gap??? function in the contact elements to implement heat transfer between the inner and outer races of the bearing. We used the thermal contact conductance to describe heat transfer between the surfaces of the bearing using the close gap function. We also determined a constant describing the thermal contact conductance for several types of bearing using a parametric simulation study, and compared the results with experimental data to determine the value of the constant. We determined the fraction of the heat flux to the inner and outer races empirically via comparisons with experimental data. Next, we carried out simulations of the rotating unit using the results of this simulation. We analyzed the thermo-mechanical behavior of the rotating unit, and compared the simulated temperature distribution and thermal deformation of the spindle tip with experimental data. We investigated three types of angular contact ball bearings, each with three different preloads. We verified the simulation results by comparing them with experimental data from a motorized rotating unit.ope

Computer numerical control vertical machining centre feed drive modelling using the transmission line technique

This study presents a novel application of the Transmission Line Matrix Method (TLM) for the modelling of the dynamic behaviour of non-linear hybrid systems for CNC machine tool drives. The application of the TLM technique implies the dividing of the ball-screw shaft into a number of identical elements in order to achieve the synchronisation of events in the simulation, and to provide an acceptable resolution according to the maximum frequency of interest. This entails the use of a high performance computing system with due consideration to the small time steps being applied in the simulation. Generally, the analysis of torsion and axial dynamic effects on a shaft implies the development of independent simulated models. This study presents a new procedure for the modelling of a ball-screw shaft by the synchronisation of the axial and torsion dynamics into the same model. The model parameters were obtained with equipments such as laser interferometer, ball bar, electronic levels, signal acquisition systems etc. The MTLM models for single and two-axis configurations have been simulated and matches well with the measured responses of machines. The new modelling approach designated the Modified Transmission Line Method (MTLM) extends the TLM approach retaining all its inherent qualities but gives improved convergence and processing speeds. Further work since, not the subject of this paper, have identified its potential for real time application

Studies on Design of Spindle-tool System and Their Effects on Overall Milling Process Stability

High speed machining using vertical CNC milling centres continues to be a popular approach in a variety of industries including aerospace,automobile,mould and die casting etc.Chatter oscillations have significant influence in restricting the metal removal rates of the machining process.The cutting process instability or chatter is assessed by prediction of frequency response at the tool tip.Present work aims at evaluating the combined effect of a spindle-housing and tool holder on the dynamics of cutting tool by considering the flexibility of spindle unit supported on bearings.The spindle-tool is analysed by using finite element modeling using Timoshenko beam theory.The dynamic characteristics and tool-tip frequency responses are obtained without considering the cutting forces.The results are compared with receptance coupling approach and using 3D modeling in ANSYS.Further experimental modal analysis on the machining spindle of same dimensions has revealed the same dynamic modes.Using the validated FE model of the system,the effects of nonlinear bearing contact forces,spindle-tool holder interface stiffness,bearing span and axial preload, tool overhang and diameter on the frequency response and cutting process stability are studied.Optimal spindle-tool system is designed for achieving maximum dynamic stiffness.The analytically stability lobe diagrams are obtained from the real and imaginary terms of these frequency responses at the tool tip.Dynamic stability issues in helical end-milling using the two and three dimensional cutting force models are considered for the analysis.The stability boundaries are experimentally verified using the cutting tests on both CNC milling spindle and modified drilling tool spindle systems while machining Al-alloy work pieces.Vibration and sound pressure levels are also employed to assure the stability of cutting operations,while surface images are used to identify the chatter marks at various combinations of cutting parameters.Dynamic milling model is employed with the flexible spindle-tool system by considering several effects including variable tool pitch, tool run-out,nonlinear feed forces and process damping. Design and stability studies on the modified drill spindle with a custom-designed work table for milling operations allowed in understanding several interesting facts about spindle-tool systems. Some control strategies including semi-active and active methods are implemented using finite element model of the spindle-tool system to minimize the chatter vibration levels/maximize the stable depth of cut during cutting operations

Development of an ultraprecision shaping machine for manufacturing of Stavax lens molds

The production of high-precision aspheric microlenses has become increasingly difficult due to an increase in the complexity of the profile, the decrease in the lens’ size, and the demand for tighter tolerances. Machines built to fabricate these lenses generally include several expensive components due to the stringent stiffness, resolution, and bandwidth requirements necessary for proper machining. This thesis deals with reducing the cost of production by building an ultraprecision shaping machine that is comprised of three reasonably priced custom made axes that meet the requirements needed for ultraprecision machining. These three axes are (1) a flexure-based, single DOF axis driven by a voice coil actuator, (2) an inchworm axis driven by an assembly of five piezoelectric actuators, and (3) a long range fast tool servo driven by a large piezoelectric actuator. These three axes were developed individually to meet a set of requirements determined necessary for the machining of a microlens mold array in Stavax, a stainless steel variant. Each axis was designed such that it would not fail due to fatigue failure, was capable of achieving a high resolution ( 200 N/µm). The X-axis needed a range greater than 250 µm, the Y-axis needed a range greater than 3 mm, and the Z-axis needed a range greater than 35 µm. The X-axis needed to be capable of following a low frequency sine wave, while the Z-axis needed to be capable of following high frequency wave forms (200 Hz). Simulations were performed to determine if the designs would meet all the requirements set. All the designed axes have met the requirements, but only the X- and Y-axes have been manufactured for testing. Preliminary testing has shown that the X-axis has at least a stiffness of 60 N/µm in both the degrees of constraint. Movement in the parasitic directions while the axis was being actuated was also tested and showed that the only movement in the parasitic directions is when the X-axis crosses the zero point. Most likely, this is due to the electronics being used, which are also making it difficult to determine the full range of the axis and close the loop. Testing on the Y-axis has revealed that it has a stiffness of at least 125 N/µm in the direction of motion and stiffnesses between 60 N/µm and 100 N/µm in the degrees of constraint. The axis is capable of running at a speed of 150 µm/s, which is only limited by the amplifiers being used. Closed loop testing has shown that the axis is capable of 10 nm steps

Modeling method for simulation of assembly variances

High quality machine tools are necessary for industrial production of precise work-pieces. In this work a method is displayed, which allows to monitor and ensure the production of these machine tools with a constant precision and quality. This method is based on measuring of the complete friction of the linear axes by current of the servo motors, which is available in the NC controller. Thereby, a deviation between identical machine tool axes becomes obvious. This deviation allows drawing conclusions on the assembly conditions of components. At the machine tool maker’s production this measurement can be conducted and be used as reference during its life expectancy in order to supervise changes of behavior. The discussed measurements have been conducted at machine tools during start-up and at a test rig with distinct assembly failures. Furthermore, a modeling method for simulation of assembly variations is introduced. By this simulation method effects on friction can be estimated at an early stage of the product development. Hence, critical assembly steps can be determined and assessed during the design phase. This allows an improvement of assembly planning by increase of effort in critical steps, whereas it becomes possible to reduce the effort for noncritical steps.Qualitativ hochwertige Werkzeugmaschinen sind eine notwendige Bedingung zur präzisen industriellen Fertigung von Bauteilen. In der vorliegenden Arbeit wird eine Methode vorgestellt, mit der die gleichbleibende Genauigkeit und Qualität bei der industriellen Herstellung baugleicher Werkzeugmaschinen überprüft und erreicht werden kann. Diese basiert auf einer Messung der gesamten Reibung der Linearachsen über die Motorströme als in der Steuerung verfügbare Messgröße. Dabei zeigt sich eine deutliche Streuung zwischen einzelnen baugleichen Achsen, die auch Rückschlüsse auf den Einbauzustand einzelner Komponenten erlaubt. Eine solche Messung kann bereits als Referenzmessung beim Maschinenhersteller geschehen und nachfolgend beim Maschinennutzer wiederholt werden, um Veränderungen zu beobachten. Zur Verifizierung wurden sowohl an Werkzeugmaschinen beim Maschinenhersteller als auch an einem Versuchsstand, in den definierte Montagefehler eingebracht wurden, Messungen durchgeführt. Weiterhin wird eine Methode, die die Modellierung von Montagestreuungen in Simulationsmodellen ermöglicht, vorgestellt. Es wird gezeigt, dass sich mit dieser Methode frühzeitig Aussagen über die Auswirkungen von Motagestreuungen auf die Reibung treffen lassen und sich die Simulation somit für eine erste Beurteilung kritischer Montagevorgänge bereits in der Konstruktionsphase eignet. Dies ermöglicht eine verbesserte Planung der Montage, indem auf die als kritisch identifizierten Schritte mehr Aufwand verwendet werden kann, wohingegen der Aufwand für unkritische Schritte gegebenenfalls reduziert werden kann

A hybrid type small 5-axis CNC milling machine

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2002.Includes bibliographical references (leaves 116-119).5-axis CNC milling machines are important in a number of industries ranging from aerospace to consumer-die-mold machining because they can deliver high machining accuracy with a spindle tilting capacity. Most of these machines have serial mechanisms so that low static and dynamic stiffness become very critical design issues especially for high speed machining. Parallel mechanisms have recently received attention from machine tool designers because of their inherent potential for stiffness and because of their compactness. However, much of the promised advantages of parallel machines only occur within a very small region of their workspace with the expense of the large machine-tool foot print. We discuss some of the kinematic and structural challenges to extracting machining performance from serial and parallel machines. We compare a hybrid machine, which combines serial and parallel mechanisms, with typical serial and parallel machines such as Euler angle machines and the Hexapod. In particular, we consider singularities, reversal characteristics, and manufacturability. We show that hybrid machines can benefit from the advantages of serial and parallel mechanisms while avoiding most potential pitfalls of both mechanisms. However, hybrid structures can suffer from the manufacturing problem of over-constraint. We show that the degree of over-constraint depends on the size of the parallel machine. We have designed and fabricated a small hybrid 5-axis motion platform, the MIT-SS-1, which can tolerate this over-constraint through a novel layout of axes. Numerical and experimental test results of the MIT-SS-1 are presented and compared. Finally we show that this structure has potential as a small 5-axis CNC milling machine.by Seung-Kil Son.Ph.D